1. Field of the Invention
The present invention concerns an arrangement a basic field magnet and a gradient coil of a magnetic resonance apparatus of the type wherein the basic field magnet is a superconducting magnet having a cryoshield.
2. Description of the Prior Art
In magnetic resonance apparatuses a measurement subject is exposed to a strong, constant magnetic field in order to achieve an alignment of the nuclear spins of the atoms in the measurement subject. Ordered or aligned nuclear spins can be excited to an oscillation (resonance frequency) by radiated radio-frequency energy. A radio-frequency response signal, which is acquired with an acquisition coil for later analysis, is excited via the oscillation.
Exact information about the respective origination location (spatial information) of the RF response signals is a mandatory requirement for the image reconstruction. This spatial information is produced by magnetic fields (magnetic gradient fields) along the three spatial directions in addition to the static magnetic field. The gradient fields are lower in magnitude in comparison to the main field and are formed by gradient coils.
Gradient coils for magnetic resonance apparatuses essentially include three-axis magnetic field coils. Typically a pulse-shaped current on the order of several hundreds of amperes (at electrical voltages of 2 kV) flows through each gradient coil. Due to the ohmic resistance of the coil, a considerable amount of energy is converted into heat that must be dissipated to avoid an overly severe heating of the gradient coil and of the inner chamber of the magnetic resonance apparatus in which the patient lies.
FIG. 4 shows the basic design of a central part of a typical magnetic resonance apparatus.
A basic field magnet GFM of the magnetic resonance apparatus includes primary and secondary superconducting coils SS that are arranged for cooling in a sealed reservoir BEH with liquid helium.
The helium reservoir BEH is surrounded by a further reservoir that, for example, is shaped like a kettle and is, for example, produced from stainless steel. This kettle-shaped reservoir is known as an “outer vacuum chamber” (OVC).
Vacuum predominates between the outer vacuum chamber OVC and the helium reservoir BEH. Cryoshield KRY is additionally arranged between the outer vacuum chamber OVC and the helium reservoir BEH.
In the case of an outer vacuum chamber OVC formed of electrically-conductive material, effects of gradient coil scatter fields on the cryoshield KRY are reduced due to the conductivity.
A cylindrical gradient coil GS is fixed concentrically in the inner chamber of the basic field magnet GFM by supporting elements, with the gradient coil GS being attached on a supporting tube.
In the gradient coil current rise rates on the order of 2500 kA/s are achieved by switching of currents in the gradient coil or in the gradient system.
Due to the switching, Lorentz forces that generate strong mechanical oscillations occur by interaction with the strong magnetic field of the basic field magnet. All system components coupled to the gradient system (such as, for example, housing, coverings, parts of the basic field magnet, etc.) are thereby excited to oscillate (vibrate).
Eddy currents are likewise generated by the pulsed fields of the gradient coil in conductor structures that surround the gradient coil. Due to interaction with the basic magnetic field, these eddy currents excite forces that act on the structures and likewise cause these structures to oscillate (vibrate).
Due to the oscillations, in operation of the magnetic resonance apparatus a strong airborne sound is generated that, as noise, disturbs the patient, the operating personnel and other people in proximity to the system.
Moreover, due to the vibrations of the gradient coil and of the basic field magnet as well as the transfer of the vibrations to RF acquisition antenna and patient bed, images of insufficient clinical (diagnostic) quality are obtained, that exhibit “ghosting”.
An artifact known as “Epi N/2 Ghosting” occurs when forces and therewith movements in highly conductive layers are caused by magnetic fields, for example in the cryoshield. KRY
Secondary eddy currents, whose field effect disrupts the imaging and that also lead to helium volatilization arise due to this movement. This is known as “helium boil-off”, namely the liquid helium is elevated in temperature and vaporized by ohmic heating. Due to the volatilization it is necessary to refill a corresponding quantity of liquid helium at high cost.